The nucleus of eukaryotic cells is enclosed by a double membrane, the nuclear envelope (NE). In order to allow the bidirectional exchange of macromolecules between nucleus and cytoplasm, the NE is perforated by numerous pore complexes (NPCs). Interphase cell growth is accompanied by nuclear growth and expansion of the NE, concomitant with an increase in the number of NPCs. The de novo formation of NPCs requires the local fusion of the inner and outer nuclear membrane along with the ordered assembly of hundreds of NPC proteins (nucleoporins) that stabilize the resulting membrane channel and regulate transport processes. We are studying processes and mechanisms involved in NPC assembly by using a cell-free system based on Xenopus egg extract.